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Mg-doped p-type a-plane GaN films were grown on unintentionally doped a-plane GaN templates by metalorganic vapor phase epitaxy (MOVPE). The Mg concentration in a-plane GaN increased with increasing Mg source gas flow rate. A maximum hole concentration of 2.0 × 1018 cm-3 with a hole mobility of 4.5 cm2/Vs and resistivity of 0.7 Ω·cm were achieved. The activation ratio was 5.0 × 10-2. It was found that a maximum hole concentration in p-type a-plane GaN was higher than that in p-type c-plane GaN. The activation energy of Mg acceptors in p-type a-plane GaN with the maximum hole concentration was found to be 118 meV by temperature-dependent Hall-effect measurement.
The sublimation growth technique is highly attractive as a commercially viable GaN substrate technology on account of its simplicity and relatively high growth rates. Sublimation growth of GaN using GaN powder source, however, is hampered by formation of liquid Ga in the source. To overcome this limitation, an oxide transport process using a mixture of gallium oxide (Ga2O3) powder and graphite powder as precursors with nitrogen gas as carrier and ammonia as the source of nitrogen has been developed. GaN layers grown by this process were studied by optical microscopy, synchrotron white beam x-ray topography (SWBXT) and high resolution x-ray diffraction (HRXRD) to characterize their structural properties. Studies reveal that the GaN layers grown are single crystal but characterized by dislocation densities and impurities higher than those obtained using GaN powder source. Observed defect distribution is correlated with growth conditions to deduce optimal growth procedure.
The performance of InGaN and AlGaN-based blue (465nm) and deep ultraviolet (UV) (280 nm) light-emitting diodes (LEDs) at elevated temperatures (25-175 °C) were investigated. As a result of uniform high-Al content AlGaN alloys yielded by migration-enhanced metalorganic chemical vapor deposition, the deep-UV LED showed dominant band-edge emission, much smaller alloy broadening and weaker localization effects as compared to the InGaN LED. Strong carrier localization was retained in the blue LED up to 175 °C, leading to temperature-independent emission intensity at low-energy tails. The UV LED, however, showed a much more rapid decrease in light output with increasing temperature. The characteristic temperature was 37 K, compared to 270 K for the blue LED. These findings implicate the lack of localization effects in AlGaN alloys as one of the causal factors in the poor thermal performance of the deep UV LED and suggest that increasing carrier confining potentials will provide a critical means to improve its thermal stability.
We report on the ultraviolet photoluminescence (UV-PL), Raman and structural properties of wurtzite MgxZn1-xO nanopowders of average size ∼ 30 nm that were synthesized via the thermal decomposition method. For the studied composition range of, the room temperature UV-PL was found to be tuned by ∼ 0.24 eV towards the UV-spectral range, and the PL emission was established to be due to an excitonic-type recombination mechanism. The first-order LO Raman mode was found to exhibit a blueshift of ∼ 33 cm-1. The LO-mode of the nanopowders is discussed in terms of a mixed A1-E1 symmetry phonon known as a quasi-LO mode. The observed 30 cm-1 blueshift indicates that the E1 is the principle symmetry component in the Raman scattering of the MgxZn1-xO nanopowders.
We present comparative micro-photoluminescence measurements on ensembles and isolated single GaN nanocolumns. The samples were prepared in a top-down approach by etching compact GaN layers grown on Si(111) and sapphire (0001) substrates. The comparison of the spectral energy change of the donor-bound exciton emission of GaN volume material and nanocolumns prepared on different substrates as well as from nanocolumns detached from the substrate gives an insight into the strain induced by the substrate. Experimental evidence for the relaxation effects were found. A common D°X spectral position at 3.473 eV was found for all detached single GaN nanocolumns independent of the substrate used, as expected for a relaxed system. Furthermore the optical properties of structural-defect related emission peaks were investigated for single nanocolumns.
An optimized packaging configuration for high-power white-light-emitting diode (LED) lamps that employs a diffuse reflector cup, a remote phosphor and a hemi-spherically-shaped encapsulation is presented. Ray tracing simulations for this configuration show that the phosphor efficiency can be enhanced by up to 50% over conventional packages. It is experimentally shown that dichromatic LED lamps with remote phosphor and diffuse reflector cup configuration have higher phosphor efficiency by 15.4% for blue-pumped yellow phosphor and by 27% for ultraviolet-pumped blue phosphor over conventional packages. Those improvements are attributed to reduced absorption of the phosphorescence by the LED chip and the reduction of deterministic optical modes trapped inside the encapsulant.
An alternative scheme to the growth of crack free, dislocation reduced III-Nitride layers on Silicon substrate has been previously introduced that relies on formation of an ion implanted defective layer in the substrate with implantation taking place in the presence of AlN buffer layer. Here, the effects of N+ ion implantation of AlN/Si (111) substrate on the structural and optical properties of the overgrown GaN epilayers have been investigated. Temperature dependent photoluminescence has been used to investigate the impact of the implantation conditions (energy and dose) on optical and structural quality of the GaN overgrown layers. A correlation between PL and high resolution x-ray diffraction (XRD) of the overgrown GaN layers show that the lowest FWHM of bandedge, the highest bandedge to deep defect blue luminescence band ratio, and the lowest symmetric rocking curve FWHM are achieved for the optimized implantation conditions. This correlates well with the results of etch pit density measurement showing an order of magnitude reduction in threading dislocation defect density
Optical and structural properties of in situ Cu doped GaN thin films grown on sapphire substrates were optically investigated by means of Raman, photoluminescence (PL), and absorption spectroscopy. Different Cu concentrations in the films were analyzed by secondary ion mass spectroscopy (SIMS) and found to vary from 2×1016 cm-3 to 5×1017 cm-3. Raman studies confirmed high crystalline quality of GaN:Cu with no major structural damages due to Cu incorporation. PL investigation revealed that the origin of the emission around 2.4 eV is most likely due to Cu incorporation. The electrical conductivity of the samples was analyzed by Hall measurements and the found semi-insulating behavior was assigned to the compensation of intrinsic donors by the deep Cu acceptor states.
Here we present results on the first atomic simulation of the threading (a+c)-mixed dislocation cores in wurtzite GaN. These calculations are based on a modified Stillinger-Weber potential. For this dislocation two core configurations are shown to be stable, one with a complex double 5/6-atoms rings and the other a with 5/7-atom rings structures. The two cores contain neither wrong nor dangling bonds.
Nonpolar a-plane GaN films were grown on r-plane sapphire substrates by atmospheric metalorganic vapor-phase epitaxy. The as-grown layers were studied by high-resolution x-ray diffraction. The a-plane GaN lattice mosaic is orientation dependent as determined by x-ray rocking curve (XRC) measurements. The tilt mosaic measured with the c-axis within the scattering plane (c-mosaic) was greater than the mosaic measured with the m-axis within the scattering plane (m-mosaic). The mosaic along both azimuths decreased and the c-mosaic/m-mosaic ratio was increased with increase of growth temperature from 1050 °C to 1080 °C. The morphological transition was correlated to change in the a-plane GaN tilt mosaic measured by XRC.
Optical properties of tensile strained AlxGa1-xN films of AlxGa1-xN/GaN heterostructures grown on sapphire were investigated by using polarization-resolved photoluminescence spectroscopy. Emissions from AlxGa1-xN with polarization of E//c and E⊥c were obtained at different peak energies. The energy separation of these emissions with polarization was increased linearly with the increase in Al mole fraction of the strained AlxGa1-xN, indicating that the energy separation was due to biaxial strain in the tensile strained AlxGa1-xN.
This paper reviews of some of the progress made in the development of ZnO-based light emitting diodes (LEDs). n-ZnO/p-AlGaN-based heterostructures have been successfully for the fabrication of UV emitting LEDs that have operated at temperatures up to 650K, suggesting an excitonic origin for the optical transitions. RF-plasma-assisted molecular beam epitaxy has been used to grow epitaxial CdxZn1-xO films on GaN/sapphire structure. These films have a single-crystal wurtzite structure as demonstrated by structural and compositional analysis. High quality CdxZn1-xO films were grown with up to x=0.78 mole fraction as determined by RBS and SIMS techniques. Optical emission ranging from purple (Cd0.05Zn0.95O) to yellow (Cd0.29Zn0.71O) was observed. Compositional fluctuations in a Cd0.16Zn0.84O films were not detected by spatially resolved CL measurements, although intensity fluctuation with features of ∼0.5 μm diameter were seen on the intensity maps. Time resolved photoluminescence shows multi-exponential decay with 21 psec. and 49±3 psec. lifetimes, suggesting that composition micro-fluctuations may be present in Cd0.16Zn0.84O film.
Reflection high-energy electron diffraction total-reflection-angle x-ray spectroscopy (RHEED-TRAXS) uses high-energy electrons from RHEED to excite x-ray fluorescence. Monitoring characteristic x-rays of selected elements thus allows study of surface coverage of materials. In this study, surface coverage of Ga and In during growth of GaN and InGaN was probed using this technique. Evolution of the surface layer of Ga on GaN during growth and deposition of Ga on static GaN at room temperature were studied. RHEED-TRAXS measurements were performed during growth of InGaN by measuring the ratio of the In Lα to Ga Kα intensity. A significant surface coverage of In was observed at all temperatures investigated regardless of actual In incorporation.
We investigated an AlGaN/GaN Schottky barrier diode (SBD) with a field plate structure for a high breakdown voltage. The AlGaN/GaN heterostructure was grown by MOCVD. The AlGaN buffer was grown on the Si (111) substrate and Al0.25Ga0.75N (25 nm)/ GaN (1000 nm) was grown on the buffer layer. The AlGaN/GaN heterostructure without any crack was obtained. After that, a Schottky barrier diode was fabricated using an AlGaN/GaN heterostructure. In order to obtain a high breakdown voltage, a gate field plate structure was used. SiO2 was formed on the AlGaN layer using a plasma chemical vapor deposition. The Schottky electrode of Ni/Au was partially deposited on the SiO2 film towards the ohmic region. The length of field plate structure was also changed to investigate the effect. Ti/Al-silicide was used for an ohmic electrode of SBD. The contact resistance of ohmic electrodes was 8E-6 ohmcm2.
The current-voltage characteristics of an AlGaN/GaN SBD were measured. The reverse breakdown voltage of the diode was also over 1000 V and the reverse leakage current was below 1.5E-6 A/mm.
Application of SiC substrates instead of the most commonly used sapphire for the heteroepitaxial growth of III-Nitrides offers advantages as better lattice matching, higher thermal conductivity, and electrical conductivity. This namely offers interesting perspectives for the development of vertical III-Nitride devices for switching purposes. For example, an AlGaN/SiC heterojunction could improve the performance of SiC bipolar transistors. In this work, n-type GaN layers have been grown by MOVPE on p-type 4H-SiC substrates using Si doped Al0.08Ga0.92N or Al0.3Ga0.7N nucleation layers. They have been characterized with temperature dependent current-voltage (I-V-T), capacitance-voltage (C-V) techniques and transmission electron microscopy (TEM).
We present a combined Capacitance-Voltage (C-V), Deep Level Transient Spectroscopy (DLTS) and Photocurrent (PC) study of short-term instabilities of InGaN/GaN LEDs submitted to forward current aging tests at room temperature. C-V profiles detect changes consisting in apparent doping and/or charge concentration increase within the quantum wells. This increase is correlated to dramatic modifications in the DLTS spectrum when the reverse bias and filling pulse are properly adjusted in order to probe the quantum well region. The new distribution of the electronic levels detected by DLTS could explain the observed decrease in the light emission efficiency [1,2] of the device, as the deep levels generated during the stress may provide alternative recombination paths for free carriers. The photocurrent spectra do not change in shape during stress, although their amplitude slightly decreases. This is related to a decrease of the device yield, in this photodetector configuration, with increasing aging time. Thus, we can suggest that the introduction of new defect levels in the bulk material lowers the free carrier mobility.
One of the major challenges affecting the performance of Npn AlGaN/GaN heterojunction bipolar transistors (HBTs) is the high base access resistance, which is comprised of the base contact resistance and the base bulk resistance. A novel concept is proposed to reduce the base access resistance in Npn AlGaN/GaN HBTs by employing polarization-enhanced contacts and selective epitaxial growth of the base and emitter. In addition, this technique reduces the exposed base surface area, which results in a lower surface recombination current. Such a structure would enable better performance of AlGaN/GaN HBTs in terms of higher current gain and a lower offset voltage. Theoretical calculations on polarization-enhanced contacts predict p-type specific contact resistance lower than 10-5 Ωcm2. Experimental results using transmission line measurement (TLM) technique yield specific contact resistances of 5.6×10-4 Ωcm2 for polarization-enhanced p-type contacts and 7.8×10-2 Ωcm2 for conventional p-type contacts.
Metalorganic chemical vapor deposition was employed to deposit high quality, highly uniform III-Nitride transistor structures on 100 mm diameter semi-insulating 4H-SiC substrates. Electron mobility was over 2000 cm2/Vs at room temperature. Sheet resistivity uniformity was as low as 0.75%. Typical standard deviations were about 1% in most properties including sheet resistivity, carrier concentration, mobility, and AlGaN composition. Additionally, wafers maintained their flat shape after deposition of these structures. Wafer bow and warp were typically less than 20 μm for optimized structures and <5 μm for the best wafers.
The evolution of stress during the MOCVD growth of AlN thin films on sapphire substrates under both low and high temperature conditions has been evaluated. The final stress state of the films is assumed to consist of the summation of stresses from three different sources: (1) the stress which arises from residual lattice mismatch between film and substrate i.e. that which persists after partial relaxation by misfit dislocation formation. The extent of relaxation is determined from High Resolution TEM analysis of the substrate/film interface; (2) the stress arising from the coalescence of the 3D islands nucleated in this high mismatch epitaxy process. This requires knowledge of the island sizes just prior to coalescence and this was provided by AFM studies of samples grown under the conditions of interest; and (3) the stress generated during post-growth cooling which arises from the differences in thermal expansion coefficient between AlN and sapphire. The final resultant stress, comprising the summation of stresses arising from these three sources, is found to be tensile in the sample grown at lower temperature and compressive in the sample grown at higher temperature. These results are in general qualitative agreement with results of TEM and High resolution X-ray diffraction (HRXRD) studies, which show evidence for tensile and compressive stresses in the low temperature and high temperature cases, respectively.
Optoelectronics related to GaN-based semiconductors (i.e., InGaN, GaN, and AlGaN) are new technologies that have the potential to far exceed the energy efficiencies of incandescent and fluorescent lighting sources. Among the GaN-based optoelectronic devices like light emitting diode (LED) and laser diode (LD), the GaN-based LEDs are of interest for the next generation illumination because of the representative characteristics such as small, highly radiant, reliably long, and fast responding, compared with the existing general lighting systems.
Achievement of high luminous intensity by flip-chip LED (FCLED) with Ag-metallic reflector or using top emitting LED (TELED) with highly transparent ITO contact is required to improve the external quantum efficiency (EQE) and light output of GaN-based LEDs. However, since the work function of Ag and ITO is lower than 5.0 eV, it is difficult to produce low-resistance p-ohmic electrode with Ag-metallic reflector or ITO only.
In this study, in order to develop new ohmic contact materials having low contact resistance and high transmittance, transparent nanoparticles-embedded p-ohmic electrode, Mg-doped indium oxide (MIO) (3 nm)/indium tin oxide (ITO) (400 nm) ohmic contact for high brightness TELEDs for solid-state lighting, was suggested. The MIO/ITO contact become ohmic with specific contact resistances of 2.64 × 10-3 Ωcm2 and give transmittance higher than 94.6 % at a wavelength of 450 nm when annealed at 630 °C for 1 min in air. GaN-based LEDs fabricated with the annealed MIO/ITO p-contact layer give a forward-bias voltage of about 3.38 V at injection current of 20 mA. It is further shown that the output power of the LEDs with the MIO/ITO contact is enhanced by about 1.86 times at 20 mA as compared with that of LEDs with the conventional Ni/Au contact.